3 Special Morphology and Classification. 287 
like incrustations on stones and the bark of trees (Fig. 410). 
The chief peculiarity of their thallus consists in the nature 
of their gonidia, which are often united into many-celled 
alga-like rows of ceils (Fig. 411), increasing in length by 
division of the terminal cell. 
The thallus of the Geélatinous Lichens (Fig. 412) has a 
leaf-like or an arborescent form, or consists of granules 
which constitute an incrustation. When dry it is cartila- 
ginous or brittle, and then absorbs water eagerly, swelling 
up into a gelatinous body. Sections (Fig. 412 1.) show that 
i : II. 
@ 
I Ag0%- 
cath e\ \ 
Og-2 
Fic. 412.—I. Longitudinal section through the thallus of a Gelatinous Lichen, Mac- 
lotinnt Hildenbrandii (x 190); II. piece of a very thin section through the 
under side with moniliform chains of gonidia (after De Bary, x 390). 
it consists of gonidia and narrow filaments of cells imbedded 
in an apparently homogeneous jelly. The contents of the 
cells are always colourless and invisible, and apparently 
contain no organised constituents such as grains of starch. 
The cell-walls of many Lichens (as, ¢.g., the so-called ‘ Ice- 
land moss,’ Cetrarta tslandica), swell up, when boiled in 
water, into a homogeneous jelly, forming the so-called 
‘moss- or lichen-starch,’ or Lichenin. 
Among the organs of reproduction of Lichens are the — 
soredia (Fig. 409 Iv.), which are developed in the gonidial 
